Fix intermittent time-based failure in delayed sidekiq spec

This example was previously failing when it was run at exactly an "even"
10 second increment time. For example, it would fail at "12:00:10", but
pass at "12:00:09" and "12:00:11". This leads to intermittent failures
on CI (presumably around ~10% of runs).

Updated to use a helper method in the spec which more closely mirrors
the scheduler code.

spec/chewy/strategy/delayed_sidekiq_spec.rb

@@ -47,7 +47,7 @@
           expect(Sidekiq::Client).to receive(:push).with(
             hash_including(
               'queue' => 'chewy',
-              'at' => (Time.current.to_i.ceil(-1) + 2.seconds).to_i,
+              'at' => expected_at_time.to_i,
               'class' => Chewy::Strategy::DelayedSidekiq::Worker,
               'args' => ['CitiesIndex', an_instance_of(Integer)]
             )
@@ -62,6 +62,11 @@
           end
         end
       end
+
+      def expected_at_time
+        target = described_class::Scheduler::DEFAULT_LATENCY.seconds.from_now.to_i
+        target - (target % described_class::Scheduler::DEFAULT_LATENCY) + described_class::Scheduler::DEFAULT_MARGIN.seconds
+      end
     end
 
     context 'with custom config' do